Commit: 1472775 (kvm: svm: Add support for additional SVM NPF error codes) added new error code to aid nested page fault handling. The commit unprotect (kvm_mmu_unprotect_page) the page when we get a NFP due to guest page table walk where the page was marked RO. Paolo highlighted a use case, where an L0->L2 shadow nested page table is marked read-only, in particular when a page is read only in L1's nested page table. If such a page is accessed by L2 while walking page tables it can cause a nested page fault (page table walks are write accessed). However, after kvm_mmu_unprotect_page we may get another page fault, and again in an endless stream. To cover this use case, we qualify the new error_code check with vcpu->arch.mmu_direct_map so that the error_code check would run on L1 guest, and not the L2 guest. This would restrict it avoid hitting the above use case. Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> Cc: "Radim Krčmář" <rkrcmar@xxxxxxxxxx> Cc: Thomas Lendacky <thomas.lendacky@xxxxxxx> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx> --- See http://marc.info/?l=kvm&m=150153155519373&w=2 for detail discussion on the use case and code flow. arch/x86/kvm/mmu.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c index 9b1dd11..4aaa4aa 100644 --- a/arch/x86/kvm/mmu.c +++ b/arch/x86/kvm/mmu.c @@ -4839,7 +4839,8 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code, * Note: AMD only (since it supports the PFERR_GUEST_PAGE_MASK used * in PFERR_NEXT_GUEST_PAGE) */ - if (error_code == PFERR_NESTED_GUEST_PAGE) { + if (vcpu->arch.mmu.direct_map && + (error_code == PFERR_NESTED_GUEST_PAGE)) { kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2)); return 1; } -- 2.9.4
